The present invention mainly relates to galvanized steels for use in a vehicle body, and more particularly, to a galvanized steel for use in outer plates of a vehicle body that has improved corrosion resistance and workability. In this specification, a vehicle typically implies an automobile.
Requirements concerning corrosion resistance and workability of steel plates for use in vehicle bodies are becoming more demanding. In respect of corrosion resistance, pitting presents a significant problem. Pitting often occurs in a door hem portion, which is the area where steel plates meet together. Since paint is not applied to this area in general, corrosion resistance without coating is particularly important for the steel plate in this area. In order to improve the corrosion resistance therefor, a specific type of coated steel plate is widely used, wherein the plate is plated with Znxe2x80x94Ni alloy to form a relatively thin (20 to 30 g/m2) layer and has an additional chromate or organic coating formed thereover. Although the thus obtained steel plate has sufficient corrosion resistance and workability, the steel plate suffers a problem that the insulative organic coating layer formed as the outer layer often causes unevenness in painting when the plate is painted by means of electro-deposition. This makes it difficult to achieve uniform appearance in terms of painting. Another problem with such plates is that they use expensive nickel and contain hazardous chromium (VI). Galvanized steels with increased amounts of pure zinc, or galvanized steels with Znxe2x80x94Fe alloy are also available. Although increasing the applied amount of plating can improve corrosion resistance of steel plates, it generally decreases their workability. Thus, it is extremely difficult to balance the two properties.
In an effort to overcome the above-described problems, the present inventors have previously proposed a method of forming an outer layer on a galvanized steel, or on a galvanized steel which has a layer of a zinc-phosphate containing synthetic coating formed thereon, by applying an aqueous solution of magnesium dihydrogenphosphate and subsequently drying the applied solution to form the outer layer. Although good corrosion resistance as well as improved workability can be obtained through this approach, it may not be ideal, given that the plates are intended for use in outer plates of vehicles. Namely, regarding the outer plates for vehicle bodies, different properties are required for the surface that serves as an outer surface of a vehicle body from the surface that serves as an inner surface of the vehicle body: High corrosion resistance to prevent the above-described pitting problem is generally required for inner surfaces while a match with paints and chipping resistance are more important than the corrosion resistance on outer surfaces. While the method previously proposed by the present inventors provides the plates with sufficient pitting or corrosion resistance, it may not provide sufficient chipping resistance, depending on conditions under which the vehicles are painted or subjected to use.
The present invention addresses to solve the above-mentioned problems. Accordingly, it is an object of the present invention to provide a coating structure suitable for use with a steel plate for outer plates of vehicles, the coating having corrosion resistance and workability that are well-balanced.
In one aspect, the present invention provides a galvanized steel for use in a vehicle body, including a galvanized steel plate having a zinc coating plated on both surfaces thereof; a zinc phosphate coating formed on one of the surfaces of the plate that serves as an outer surface of a vehicle body; and a phosphate-containing composite coating formed on the other surface of the plate that serves as an inner surface of a vehicle body, the composite coating being composed of a zinc phosphate coating layer and a phosphate coating layer containing Mg. The phosphate-containing composite coating preferably contains 2 wt % or more of Mg, and is preferably applied to the plate in an amount greater than or equal to 0.5 g/m2.
The present invention will now be described in detail by exemplary examples which are to be construed as illustrative, rather than restrictive.
Galvanizing processes used in the present invention is not specifically limited, and both pure zinc galvanization and alloy galvanization can be adopted to take advantages of their capability to provide good corrosion resistance and improved workability. Galvanizing processes such as electrogalvanizing, hot dipping, or alloy hot dipping are particularly preferred in terms of manufacturing cost. Also, galvanization may be either single-layered or multiple-layered, or it may be applied over a pre-plated layer formed of Ni, Cu, or the like.
A layer of zinc phosphate coating is formed on a galvanized steel on each of the opposite surfaces of the plate, one surface serving as an inner surface of a vehicle body and the other surface serving as an outer surface of a vehicle body. Zinc phosphate coating used to form a layer over the galvanization layer of the steel plates may be those that are commonly used, and the coatings can be formed by using commercially available treatment solutions containing zinc ions, phosphate ions, or the like. The amount of the zinc phosphate coating applied is preferably in a range from about 0.3 g/m2 to about 2 g/m2 under normal conditions. When the coating is applied in an amount less than the lower limit of the range, the corrosion resistance and workability may become insufficient, whereas the amount exceeding the upper limit of the range often makes welding of the plates difficult. Since zinc phosphate coating is typically applied by dipping or spraying, it is difficult to control the amounts of the coating so that the coating is applied in different amounts on the outer surface and the inner surface of the plate. Though the same amount of the coating may be applied to each surface of the plate, a smaller amount (e.g., 0.1-1.5 g/m2) is preferably applied to the surface that serves as an outer surface of a vehicle body than the amount applied to the other surface of the plate that serves as an inner surface of a vehicle body when it is possible to control the amounts of coatings applied, for example, by separately spraying onto each surface, so that different amounts of coating are applied on the opposite surfaces. Even for outer surface, however, it is undesirable to apply no coatings as it not only makes welding difficult but also reduces workability due to the difference in slidability between the opposite surfaces. At least about 0.1 g/m2 of the coating needs to be applied for outer side.
In terms of corrosion resistance and workability, the zinc phosphate coating preferably contains one or more selected from the group consisting of Ni, Mn, Mg, Co, Ca, Cu, and Al. In such a case, steel plates are treated in a bath of a zinc phosphate treatment solution containing the metal ions described above. Also, the composition of the zinc phosphate coating applied to the surface that serves as an outer surface of a vehicle may or may not be the same as that of the zinc phosphate coating applied to the other surface of the plate that serves as an inner surface of the vehicle.
A phosphate coating containing Mg can be formed on the surface that serves as an inner surface of a vehicle body by applying an aqueous phosphate solution containing Mg over the above-mentioned zinc phosphate coating and subsequently drying the solution. This results in the formation of a composite phosphate coating composed of the zinc phosphate coating and the Mg-containing phosphate coating laminated on the zinc phosphate coating. As a result, excellent pitting and corrosion resistance is achieved on the surface that serves as an inner surface of a vehicle body. The coating which is formed by applying an Mg-containing aqueous phosphate solution followed by drying the applied solution, is necessary only on the surface that serves as an inner surface of a vehicle body and not essential on the surface that serves as an outer surface of a vehicle body. Formation of the above-mentioned coating on the outer surface may reduce the chipping resistance of the surface. The coating may preferably be applied to the plate surface that serves as an inner surface of a vehicle body in an amount greater than, or equal to, 0.5 g/m2, which is the total amount in the composite phosphate coating which is composed of the zinc phosphate coating and the coating formed by applying an aqueous phosphate solution containing Mg and then drying it. A preferred content of Mg in the composite phosphate coating is 2 wt % or more. Good corrosion resistance is achieved when these conditions are met. The maximum amount of the composite phosphate coating to be applied, as a total, is preferably 2.5 g/m2 or less in terms of workability.
Preferably, an aqueous solution of Mg(H2PO4)2 is used as the Mg-containing phosphate solution. A commercially available solution of magnesium dihydrogenphosphate (e.g., available from Yoneyama Kagaku Kogyo Co., Ltd.) is most preferably used as the Mg-containing phosphate solution for its wide application. These solutions are applied only to one surface (i.e., inner surface) with, for example, a roll coater(coating roller) and are subsequently dried to form a composite phosphate coating.